Hi guys,
it shouldn't be a matter whether you specify the geometry for the new row, but there are some extra steps you might want to take:
1. Use the "using" keyword for the FeatureDataTable, (but you don't need one for MobileCache)
2. After a new Row is created, call row.BeginEdit() to start and row.EndEdits() to close the editing session.
A work around for the issue is to create a new record using a FeatureDataReader instead of a FeatureDataTable. again it will be good if you use the using keyword for the FeatureDataReader.
If still you cannt save locally you might want to check if it's something happening with the layer/cahe itself. You might want to check if your cache is valid after opening it.
and for more checking (that's just my usual practice), you might want to check if your FeatureDataTable has the correct number of rows before you add any new row. since the table itself might have problems.
Please let us know if that helps!
Tif